Rotational molding, more commonly known as rotomolding, is generally used for molding hollow articles such as outdoor sports equipment, storage tanks and refuse containers. To rotomold a part, powdered polyethylene or other plastic resin is placed inside a hollow mold, which is then closed and rotated on two axes, inside a heated oven to allow the powder to fuse together, forming a hollow mass inside of the mold. The mold is removed from the oven to cool, and after cooling, the molded part is removed. Thereafter, more powder is placed inside the mold to begin another cycle. A more detailed discussion of rotomolding may be found in Modern Plastics Encyclopedia 1990, pages 317-318.
Foaming agents are useful to improve appearance, reduce part weight, provide a higher insulation value, increase stiffness and reduce resin cost. Generally, foaming agents may be classified as "physical" or "chemical" foaming agents. Typically, physical foaming agents are normally gaseous at the temperature at which the plastic mass is to be foamed. Chemical foaming agents are typically compositions which decompose or react to form a gas. Heat decomposable, foaming agents include organic compounds such as azodicarbonamide, 4,4'-oxybisbenzenesulfonyl hydrazide, and p-toluenesulfonyl hydrazide; disadvantages include cost and unpleasant odor. Carbon dioxide-releasing, foaming agents include inorganic salts such as sodium, ammonium, and potassium bicarbonate. A foaming coagent is advantageously used with a reactive type, chemical foaming agent, and for an acid-released, foaming agent, a fatty acid such as stearic acid or a mild organic acid such as citric acid is often used as the coagent.
A useful additive when foaming thermoplastics is often a nucleating agent. A nucleating agent ensures uniform cell size and reduces the existence of surges and voids in the foam. Typically, a nucleating agent remains solid or is thermally stable during foaming, and is usually finely powdered. As exemplified by U.S. Pat. Nos. 4,107,354, 4,110,269 and 4,275,168, nucleating agents useful when foaming polyolefins include calcium carbonate; silica products; pigments; carbon black; and silicates such as kaolins, mica, talc, aluminum silicate, magnesium silicate, and calcium silicate.
As illustrated by U.S. Pat. Nos. 4,110,269, 4,275,168, 5,047,440, and Re. 32,356, ethylene vinyl acetate copolymer or ionomer resin based on ethylene vinyl acetate copolymer has been used with foaming agents to produce porous structures.
Despite advances in foamable compositions, there is a need for a foamable, polyolefin-based composition suited for rotomolding applications.